The invention relates, in general, to field optical systems and more particularly to a field emission gun capable of producing an intense beam of focused particles.
In the applicant's co-pending patent application, Ser. No. 46,425, filed June 15, 1970, which issued as U.S. Pat. No. 3,678,333 there is described a scanning electron microscope employing a field emission tip as its source of electrons, which application is herein incorporated by reference. An electron microscope belongs to the family of devices known generically as microprobes, each member of which can utilize the field emission tip of the present invention. The use of the field emission tip allows the formation of a high intensity focused beam of charged particles providing the illumination needed in scanning microscopy. The electron gun of the co-pending application provides and maintains the high vacuum requirements of a field emission tip and incorporates a high degree of voltage discharge protection which, in the past, have been significant problems in the application of field emission tips to scanning electron microscopes. In implementing the high voltage protection of the field emission gun, a shield electrode placed about the tip periphery and a field electrode placed in the neighborhood of the field emission tip were added to the system. In the context of discharge protection the presence of the field electrode permits control of emission current and particle acceleration, thus returning the operation of the gun to the condition as found prior to incorporating the high voltage discharge protection shield.
It is often desirable to operate a field emission electron gun as a self-focusing electron accelerating system without the use of additional de-magnification lenses, i.e., the gun is utilized to form a focused image of the electron beam in an appropriate image plane without any additional lenses other than that provided by a main acceleration electrode and an intermediate electrode.
In prior art field emission guns employing only two electrodes, viz. intermediate and main electrodes, the self focusing mode of operation may require high extraction voltages on the intermediate electrode to achieve sufficient field strength to produce a useful level of current in the focused spot and commensurately high acceleration voltages on the main electrode to achieve acceptable focal distances. It also demanded extremely fine geometry of the field emission tip, generally a diameter of less than 1000A. If the tip exceeded this size it was necessary to place it closer to the intermediate electrode with concomitant unacceptable dislocation of the image plane.
Related to the foregoing problems, in prior art field emission guns any changes in the operational performance of the field emission tip itself such as geometrical deterioration or changes in work function of the emitting surface due to contamination manifested a significant alteration in microscope operation. The intensity of the beam as well as the location of the image plane were altered and needed system adjustment to return to an acceptable or desirable performance condition.
It is, therefore, an object of this invention to provide an improved field emission particle gun. It is another object of this invention to provide a self-focusing field emission electron gun. Another object of this invention is to provide a field emission electron gun less sensitive to variations in field emission tip performance and configuration. Yet another object of this invention is to provide a field emission electron gun producing minimum contamination in the area contiguous with the field emission tip. Yet another object of this invention is to provide a field emission electron gun capable of performing at minimum levels of focusing and acceleration voltages. It is still another object of this invention to provide a field emission electron gun in which the field emission tip is protected from high voltage discharges.